In aviation, what does "crosswind" refer to?

Crosswind refers specifically to wind that blows at an angle to the direction of the runway. This aspect is particularly important during takeoff and landing, as it can significantly affect the aircraft's performance and handling. Pilots need to consider crosswinds when planning their approaches to ensure they can maintain control of the aircraft throughout these critical phases of flight.

When an aircraft is flying in a crosswind, the pilot may need to use specific techniques to compensate for the wind’s effect on the aircraft's trajectory, particularly when lining up with the runway. Such adjustments are vital for ensuring a safe landing.

The other choices describe different wind conditions that do not align with the definition of crosswind. Rather than affecting the aircraft's alignment with the runway, parallel winds or headwinds would impact the aircraft's ground speed or lift. Consistent wind at all altitudes is not a factor in defining crosswinds but rather refers to general wind characteristics that do not designate directionality relative to the runway.